1. Field of the Invention
The present invention is related to a tablet using a cordless pointing instrument, especially, to a tablet using a passive cordless pointing instrument, and any battery is not required within the cordless pointing.
2. Description of the Prior Art
In the prior art, the pointing instrument of a tablet using electromagnetic induction is installed with a coil and a capacitor to form a resonant circuit. In the active pointing instrument, a battery is needed for the resonant circuit to generate an alternating current signal which may be emitted through a coil, and a sense line grid is located under the tablet for inducing the emitted signal and then the precise position of the pointing instrument may be determined.
For example, in U.S. patent application Ser. No. 5,247,138, a logic circuit is located on the pointing pen for encoding the switch state as a 9 bit number, wherein a further controlling circuit for controlling said resonant circuit is employed so that the alternating current signal may stop transmitting for a while. Through this circuit, the emitted signal may be changed, for example, the number "1" represents as emitting signal continuously for a predetermined time, while the number "0" represents as emitting signal just half of the predetermined time, and stop emitting in the rest half predetermined time. Therefore, said 9 bit numbers may be sequentially emitted by this modulation method and then they are demodulated and decoded into a number representing the switch state, then said tablet can determine the switch state on the pointing instrument.
In the prior manufacturing method, which the passive pointing instrument is used, a coil and a capacitor is also used to form a resonant circuit, but it is needless to use any battery. On the tablet, there is a first set of parallel conducting wire to emit an alternative signal The resonant circuit in the pointing instrument will resonate with said alternative signal, the received energy may be further emitted, and a second parallel conducting wire set which overlaps on the first conducting wire set for receiving the signals emitted by the resonant circuit within the pointing instrument so to determine the precise position of the pointing instrument, since the second parallel conducting wire set is orthogonal to the first parallel conducting wire set, therefore, it will not receive the induced signal emitted by the first conducting wire set.
In U.S. patent application Ser. No. 5,220,324, a resistor and a capacitor within the passive pointing instrument are connected in serial. The capacitor in the resonant circuit is connected with said combination of said resistor and capacitor in parallel. A set of switch is used to select some predetermined resistors which connect with said resistor for changing the combined resistance. Therefore, as the resonant circuit has received the alternating current signal from the first set of parallel conductors, since the resistor will be varied, the tuning frequency of the resonant circuit will change and if the signal is further emitted to the second set of parallel conductors, the phase thereof will be changed. Thus in the tablet, a phase detecting circuit may be used to detect the phase difference between the induced signal on the second set of parallel conductors and the emitted alternating current signal. The phase difference represents the added resistor, i.e. the number of switches that are pressed by the user.
In summary, there are some disadvantages in the U.S. Pat. No. 5,247,138:
1. The battery is needed to operate within the pointing instrument, thus the weight must be increased and the battery life is also limited.
2. A complicated modulating and encoding circuit are required, especially, if the switch input is an analog signal, for example the pressure sensor on the pen tip, then an analog to digital converter will be employed, therefore, the power consumption is getting so high and the battery life will shorten.
3. Since the data transmitting timing on the pointing instrument is not synchronous with the position scanning timing of the tablet. Thus it is possible that when the tablet is inducing the signal for determining the pointing instrument position, the emitted signal may be intermittent because the data transmitting. Thus the induced signal will not be steady, of course the determined position of the pointing instrument is also unstable.
Another drawback in the U.S. Pat. No. 5,247,138 is that in general a metal plate is installed under the tablet for shielding the external noise. But if the pointing instrument is located on the tablet when using, it will close to said metal plate, but once the coil is near the metal plate, the inductance of the coil thereof will change. Therefore, even if the switch has not pressed, once it is close to the metal plate, the phase of the receiving signal will vary which depends on the distance. Although in the patent application, by employing a lookup table, the induced signal may be used to correct the phase. But in practical manufacturing, the material of the material plate and coil, and the tightness of the coil has large effect, thus they are needed to match with each other so to meet the original designed look up table in the ROM. Therefore, in manufacturing, many variable factors are generated, thus multiple correction operations are needed, so it is hard to do the mass production.